PROCESS AND INSTALLATION FOR REPAIRING THE LINING OF STEELMAKING CONVERTERS

Abstract: Process and installation for repairing the refractory surface of a converter (1), in which the slag therein contained is moved to make it splash against defined zones of the refractory walls (8) of the converter (1), said movement being obtained by means of a gas flow blown under the surface of the slag through a plurality of conduits (7) placed within the refractory walls (8).

Description: Process and installation for repairing the lining of steelmaking converters FIELD OF INVENTION The present invention refers to a process for repairing the internal refractory surface of steelmaking converters. The invention also refers to the plant permitting to realize such process. PRIOR ART In steelmaking, a converter is a vessel containing molten metal (pig- iron, steel) to be treated to ; for instance decarburize and/or de- sulphurize it and the like. The converter comprises an outer metallic vessel, or shell, internally lined with refractory materials. The shell bears a couple of diametrically opposed supports, or trunnions, utilized to move the converter. The plane defined by the rotation axis of the trunnions and by the converter longitudinal axis, divides the converter into two parts, one of which to charge the metal into the converter (charging side) and the other one to tap the molten steel out of the converter (tapping side) . During the operation of the converter, the refractory lining is eroded. Usually, it is completely worn-out after about 1200 to 2000 heats, after which it is dismantled and rebuilt. The cost of rebuilding is of the order of many millions of dollars, and the rebuilding is required generally every three months, for three shifts working plants, in which about 20 heats per day are made. The zones of refractory most prone to wear are those in proximity of the supports, where deep fractures are possible, which can also induce a collapse of the whole refractory structure. The charging and tapping sides are usually less prone to wearing, also because it is nowadays common to make "tarnitage" operations, consisting in protecting such zones with the slag produced during the different steps of treatment of the molten metal into the converter. A given amount of slag is left into the converter after tapping the molten metal, and the converter itself is made to oscillate around the axis of the trunnions, so that the slag is adhered to the refractory in correspondence of the charging and tapping sides. During such operations, however, the zone corresponding to the trunnions obviously is not covered by the slag. To obviate this problem, the current technique is to spray into the converter a mixture of magnesite powder and water, starting after about 1000 heats. However, due to the low adhesion characteristics of the magnesite-water mixture, it is necessary to repeat this tratment after each heat. The results of this treatment are unsatisfactory and the waste of time and resources can be high. As a general rule, 1,2 kg of magnesite powder are used per ton of steel produced, and the operation requires stopping the plant. It is interesting to note, in this respect, that even short plant stops (such as for the above spraying, requiring about 5 min) , are significatively comparable to the duration of a heat (about 20 min). Another process requiring to move the slag by means of high speed inert gas jets, produced by an external lance blowing on the slag surface to make the same impinge on the critical zones of the converter's walls, is believed to be insatisfactory because of its low efficiency and high costs, of the order of magnitude of remaking the refractory lining. In fact this process requires to overcome, in a costly way, particularly in the economic sense, a number of technical problems, such as: (i) building of a further line for the nitrogen gas, to guarantee the high flows and pressures necessary to move the slag from outside the converter and (ii) moving the external lance which, being rigid, could not reach all the zones to be treated within the converter (1st European Oxygen Steelmaking Congress; Duesseldorf/Neuss, June 21-23 1993, R. 0. Russel et al. 220- 225). Also the processes requiring blowing through the converter bottom present the same drawbacks connected with the external lances (high plant costs, doubling of the gas lines, difficulty to reach specific sites). SUMMARY OF THE INVENTION It has been now found, and it is an object of present invention, a process for repairing the refractory surface of converters which allows to move the slag to make it impinge on defined zones of the refractory walls of the converter. Another object of present invention is the plant through which said process can be realized. Further objects of the invention will become evident from the following detailed description. DETAILED DESCRIPTION OF THE INVENTION The repairing process according to the present invention is carried out by means of gas jets originated under the free surface of the slag floating on the liquid metal and directed to convey part of said slag toward defined zones of the refractory wall, more particularly, the internal zones corresponding to the supports. Such gas jets are erogated by at least one dedicated conduit traversing the refractory wall in the upper half of the converter. Preferably at least two conduits are used; in any case their diametre is comprised between 25 and 30 mm, preferably between 26 and 28 mm. It is also preferable that the liquid head, consisting essentially of slag, above the outlet openings of the conduits is about 25 to 30 cm; in fact, should such head be too high, the momentum of the blown gas would be dispersed into a too large mass; on the contrary, with a too small head there would be the risk to pierce the slag layer thus inadequately moving a too small mass of liquid. In some cases, therefore, it could be useful to eliminate part of the slag, for instance by means of traditional deslagging operations. To spray the slag, it is necessary to erogate from the conduits inert gas, preferably selected between nitrogen, argon and mixtures thereof, with a flow rate up to 15 Nm-^/min for times depending on the extension of the zone to be treated, preferably, at least two minutes. To avoid the slag and the liquid metal obstructing the exit openings of the conduits, during the various phases of treatment of the liquid metal, for instance during the oxygen blowing, it is advisable to let flow in the conduits a given minimum amount of inert gas, to create a protective barrier against clogging. During such phases, the flow could reasonably be of around 4-6 Nm-^/min for each conduit, with a pressure of about 3,1 bar at the inlet of the conduits and of about 1,5 bar at the outlet thereof. To send the slag toward the critical zones to be repaired, it is necessary for the conduits to be adequately inclined with angles according to the geometry and constructional parametres of each converter. Such inclination shall also be defined as a function of the design difficulties related to the realization of the conduits within the refractory walls. Therefore, to determine the inclination of each conduit, it must be taken into account also the necessity of piercing the minimum possible number of refractory bricks. It is believed to be reasonable a design requiring two conduits on the charging side of the converter, having their outlet openings under a head of 20-30 cm of slag and symmetrically disposed with respect to a vertical plane at right angle to the vertical plane containing the axis of the trunnions, and angularly spaced from one another by an angle of less than 50°, and each with its symmetry axis at an angle to the horizontal axis of the converter, originating from the intersection of each conduit with the steel shell, of less than 30°, both on the vertical and on the horizontal, said conduits being downwardly directed. Though the realization of the bores for said conduits does not present any specific problem with respect to the metallic shell of the converter, this is not the case with respect to the refractory walls. Owing to the relative motion between walls and shell, due to the different thermal expansion coefficients of said materials, there is a sliding between parts in contact to one another with ensuing misalignment between different sections of the bores. Therefor, it is advisable to utilize high toughness steel tubing. The repairing process according to the present invention can be advantageously carried out during normal deslagging operations, without slowing down nor hindering in any way the plant running. Further advantages of the invention consist in prolonging the life of the refractory by a factor of at least 3 to 5 and in limiting, and even avoiding, the use of magnesite powder, which is currently used to slow- down the refractory wear. The repairing operation according to the present invention is simple, quick and economically advantageous, in that it is carried out simultaneously to other process steps, hence without prolonging the usual tap to tap interval; moreover, it requires only the installation within the converter's refractory of small diametre tubes which do not need any auxiliary device to be moved nor dedicated inert gas lines. With respect to this point, it is to be reminded that in a steelmaking shop the inert gas lines are present and are designed for relatively low flow rates and speeds. The process according to present invention utilizes such facilities which are not apt to the known devices (external lances, conduits for blowing from the converter bottom) . A preferred embodiement of the plant necessary to carry-out the process according to present invention will now be illustrated with reference to the appended figures, which however are not to be considered as limiting the scope of the invention. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a schematic frontal view of a converter; Fig. 2 is a section at the A-A' plane of the converter of Fig. 1; Fig. 2A is an enlargement of the circled particular of Fig. 2; Fig. 3 is a section at the B-B' plane of the converter of Fig. 1. With reference particularly to Fig. 1, a converter (1) is schematically illustrated, from the charging side, comprising the following elements: the carging mouth (2), the trunnions (3, 3') and the trunnion ring (-4), for short ring. For a correct operation of the conduits according to the invention, the same are placed at 5 and 5' sites of the charging side above the trunnion ring (4) between the brackets (6) symmetrically placed above and under the trunnion ring (4). Positions (5 and 5') are chosen both to impart the right inclination to the conduits (not shown) and to have the terminal part of the same conduits within the converter (1) always in contact with the slag. The ensemble of tubes bringing the gas(es) to the conduits (not shown) is generically identified by (7). Fig. 2 shows the A-A' section of converter of Fig. 1, with trunnions (3, 3') and trunnion ring (4). Within the refractory wall (8) are traced the positions of conduits according to the invention, placed at (5, 5') and more particularly detailed in Fig. 2A. In the illustrated specific embodiement, the conduits are on a plane perpendicular to the longitudinal axis of the converter (1) with an angle of ±α with respect to the plane including said longitudinal axis and perpendicular to the axis of the trunnions (3, 3' ) • I n this specific embodiement, α is of about 16°, however, as above specified, the values both of said angle and of the conduits inclination with respect to the plane perpendicular to the longitudinal axis of the converter (1) can be changed as a function of the converter geometry. Fig. 3 shows the B-B' section of the converter of Fig. 1. The charging mouth is indicated with (2), the trunnion ring with (4), the refractory wall with (8) and the casting hole with (9). The charging side of the converter is on the left side of the Fig. 3, while the casting side is on the right side.

Claims: CLAIMS 1. Process for repairing the internal refractory surface of converters utilized in steelmaking to treat molten metal therein contained and comprising an external steel structural vessel, or shell , internally coated with a refractory material, two supports, or trunnions, being placed at diametrally opposed places on the shell, which are utilized to make the converter oscillate, said process being characterized in that a slag therein contained, deriving from the treatment of the molten metal, is displaced to make it splash against specific zones of the refractory walls of the converter, said displacement being obtained by means of a gas stream flowing through at least one conduit placed within the refractory walls of the converter and being blown, through at least one outlet opening of said conduit, under the surface of the slag floating on the liquid metal. 2. Process according to claim 1, in which the at least one outlet opening of each of said conduits is placed about 25 to 30 cm under the surface of the slag. 3• Process according to claim 1, in which there are two conduits, placed on the charging side of the converter symmetrically with respect to a vertical plane at right angle with the vertical plane containing the trunnion axis, with an angular distance therebetween of less than 50°, each having its own symmetry axis inclined with respect to the horizontal radius of the converter, originating from the intersection of each conduit with the shell, with an angle of less than 30° both on a vertical and on a horizontal plane, said conduits being directed downwards. 4. Process according to claim 3, i n which the outlet openings of the conduits are placed from about 25 to about 30 cm under the surface of the slag. 5- Process according to claim 1, in which the zones to be repaired of the refractory walls are placed corresponding to the trunnions. 6. Process according to claim 1, in which the gas flowing out of the conduits is chosen between nitrogen, argon and mixtures thereof, and has a flow rate of up to 15 Nm-^/min. 7- Process according to claim 1, in which the conduits have a diametre comprised between 25 and 30 mm. 8. Process according to claim 1, in which the conduits have a diametre comprised between 26 and 28 mm. 9- Plant for repairing the refractory walls of converters used in steelmaking for the treatment of a liquid metal therein contained, said plant comprising a converter (1) which in turn comprises: an external steel structural vessel, or shell, having sustaining brackets (6), and on its maximum circumference a trunnion ring (4), two trunnions (3, 3') placed at diametrically opposed positions on the trunnion ring (4), said vessel being internally clad with refractory material (8) , said plant being characterized in that on the charging side of the converter (1), above the trunnion ring (4), and in the space between the sustaining brackets (6), are placed at (5, 5') positions two conduits apt to let a gas flow therethrough, said conduits having relevant outlet openings under the surface of a slag floating on the liquid metal, and being angularly spaced for an angular distance of less than 50° and each having its own symmetry axis inclined with respect to the horizontal axis of the converter, originating from the intersection of each conduit with the shell, with an angle of less than 30° both on a vertical and on a horizontal plane, said conduits being directed downwards.

Inventors: BENCINI CLAUDIO, D ANDRIA CATALDO, DE CESARE ALESSANDRO, MOSCHINI AUGUSTO, BENCINI, CLAUDIO, D'ANDRIA, CATALDO, DE CESARE, ALESSANDRO, MOSCHINI, AUGUSTO
Applicants: ANDRIA CATALDO D, BENCINI CLAUDIO, CESARE ALESSANDRO DE, ILVA LAMINATI PIANI, MOSCHINI AUGUSTO, SVILUPPO MATERIALI SPA, BENCINI, CLAUDIO, CENTRO SVILUPPO MATERIALI S.P.A., D'ANDRIA, CATALDO, DE CESARE, ALESSANDRO, ILVA LAMINATI PIANI S.P.A., MOSCHINI, AUGUSTO
Application year: 1994
Main classification: C21C5
IPC codes: C21C5, F27D1